Measuring devices having a so-called “4 to 20 mA” signal use a signal output, on which a current signal is issued for communicating measured data. For this purpose, the electrical current range between 4 and 20 mA is used for the output signal. The current signal can, in such case, be injected into the current loop from a current sink or from a current source. Additionally, there is the possibility of a single-pole, or a two-pole, signal output. In the case of the single-pole variant, the current is injected into an external load resistor via an output, and the return line for the current is outside of the device. In the case of the two-pole variant, the return line is through a second pole, into the device.
The following are requirements for a suitable current source:    1. The current source injects into the external load an electrical current which is independent of the load;    2. The output current is controllable from a controller or microprocessor; and    3. The current output fulfills high requirements as regards linearity, temperature drift and stability and is, also, manufacturable cost-favorably and in high numbers of pieces.
Current sources controlled both with open-loop control and with closed-loop control are known. For an open-loop controlled current-source, components with extremely close tolerances, thus expensive components, are required. Alternatively, the applied resistors must be trimmed, an option which increases complexity and does not fit with efficient manufacture. Furthermore, drift in the components e.g. as a function of temperature, leads to undesired altering of the output current.
In the case of closed-loop-control current-sources, the output current at a measuring resistor is measured and compared with a desired value. Deviations are controlled to zero using an adjuster. A problem here, especially in the case of a single-pole current source, concerns measurement of the output signal at the measuring resistor. This measurement usually makes an amplifier necessary, via which uncertainties and fluctuations in the measurement can arise.